1. Field of the Invention
The present invention relates to an image forming apparatus and particularly to an image forming apparatus having a paper refeed tray and being capable of performing duplex copy and composite copy.
2. Description of the Prior Art
Copying apparatus capable of performing duplex copy and composite copy have been proposed in the prior art.
The duplex copy and the composite copy are copy operation as described below. After a first image is formed on one surface of paper fed to an image forming portion, the paper is not discharged outside the copying apparatus and it is temporarily stored in a refeed tray. Then, it is refed to the image forming portion and a second image is formed on the same surface of the paper (in the case of the composite copy) or on the opposite surface of the paper (in the case of the duplex copy).
In either of the two cases, a size of a paper storing portion of the refeed tray is variable to be adapted to a size of paper stored. This is because there are various sizes of paper, e.g., B4, A4 or B5-size paper, fed from paper feed portions such as paper feed cassettes although generally only one refeed tray is provided, and it is necessary to adapt the refeed tray for such various sizes of paper.
The paper size which is necessary data for determining the size of the paper storing portion of the refeed tray is detected conventionally in a paper feed cassette which stores paper to be fed.
For example, if an A4 paper feed cassette is selected as a feed opening to feed paper of the A4 size, a code signal indicating the A4 size is inputted to a control CPU of the copying apparatus. The CPU evaluates the paper size based on the code signal and a predetermined table so as to change the size of the paper storing portion of the refeed tray. Thus, detection of the paper size is effected in the paper feed cassette. For this reason, for example in manual feed copy, in which paper is manually fed from outside as required without using a paper feed cassette, the paper size cannot be detected because the paper does not pass through the paper feed cassette. Accordingly, the size of the paper storing portion of the refeed tray cannot be determined and neither duplex copy nor composite copy can be executed.
In addition, the precision of detection of paper size in the paper feed cassette is not so high. This is because the main object of paper size detection is to indicate selection of paper of any size such as A4 or B5 and not to detect an actual size of individual paper. Therefore, the detection method adopted is the method of inputting a code signal corresponding to a selected paper feed cassette. However, for example in the case of "paper of the A4 size", the size is not always the same in a strict sense and there is sometimes a little error. In addition, it happens that paper of a size different from the A4 size is stored in an A4 paper storing cassette. In such a case, a trouble such as paper jam in the refeed tray is likely to occur because the size of paper actually stored is different from the size based on the data for determining the size of the paper storing portion of the refeed tray.